Solar cells are photovoltaic cells or modules, which convert sunlight directly into electricity. Photovoltaic (PV) cells are made of semiconductor materials, most commonly silicon. When light (ultraviolet, visible and infrared radiation) strikes the cell, a certain portion of it is absorbed within the semiconductor material, such that the energy of the absorbed light is transferred to the semiconductor and an electrical current is produced. By placing metal contacts on the top and bottom of the PV cell, the current can be drawn off to use externally. The current, together with the cell's voltage, defines the wattage that the solar cell can produce.
A typical semiconductor photovoltaic cell comprises a large area p-n junction, where absorption of light results in the creation of electron-hole pairs. The electrons and holes migrate to opposite sides of the junction such that excess negative charge accumulates on the n-doped side and excess positive charge accumulates on the p-doped side. In order for the current to be collected for power generation, electrical contact of both sides of the pn junction to an external circuit must be made. The contacts typically consist of two metallic patterns, with one on each side, in ohmic contact with the semiconductor device. The ideal contacting pattern will have high conductivity in order to minimize resistive losses, good electrical contact to the substrate in order to efficiently collect current, and high adhesion to ensure mechanical stability. The metal pattern on the cell front-side, defined as the side of the cell which is exposed to incident light, is designed to provide a low resistance path for collecting current generated at any location on the surface of the cell and, simultaneously, to minimize the amount of incident radiation intercepted by the metal and thus lost for current generating purposes.
Silicon, especially in its crystalline form, is a common material used for producing solar cells. Most solar cells are made from crystalline silicon, doped with boron and phosphorus to produce a p-type/n-type junction. Polycrystalline silicon can be used in solar cell fabrication to cut manufacturing costs, although the resulting cells may not be as efficient as single crystal silicon cells. Amorphous silicon, which has no crystalline structure, may also used, again in an attempt to reduce production costs. Other materials used in solar cell fabricated include gallium arsenide, copper indium diselenide and cadmium telluride.
A typical arrangement of a silicon solar cell comprises as follows:                (a) a back contact usually comprising a layer of aluminum or aluminum alloy and busbars comprised of silver or a silver-aluminum alloy;        (b) a P-type Si;        (c) an N-type Si;        (d) an antireflective coating on the front-side of the cell;        (e) a front contact usually comprising a metallic grid of fingers and busbars; and        (f) a cover glass.        
Because silicon is extremely reflective, an antireflective coating is typically applied to the top of the cell to reduce reflection losses. A glass cover plate is then typically applied over the antireflective layer to protect the cell from the elements.
Conventional solar cells may be made using crystalline silicon wafers. The Si wafer starts as a p-type semiconductor with a boron dopant. To better capture light, the wafer may be texturized with hydroxide or nitric/hydrofluoric acids so that light is obliquely reflected into the silicon. The p-n junction is formed by diffusion of an n-type dopant, typically phosphorus, using vapor deposition or diffusion. A layer of dielectric is applied, typically silicon nitride or silicon oxide, to the front-side; this layer serves as both a surface passivation layer and an anti-reflective coating (ARC).
In one standard process of silicon solar cell fabrication, the front side of the silicon wafer is coated with an anti-reflective passivation layer, which typically comprises silicon nitride. This silicon nitride layer serves the dual purpose of maximizing the percentage of light absorbed by the cell (not reflected), as well as passivating the surface, which prevents electron-hole recombination at the surface and thus increases cell efficiency. A different effect is typically employed on the cell back surface to minimize electron-hole recombination. A “back surface field” (BSF) is achieved by making a so-called p+-doped layer in the silicon close to the rear surface, where the p+-doped layer contains a higher concentration of p-dopant than the bulk p-doped substrate. This creates an electric field close to the interface which provides a barrier to minority carrier (electron) flow to the rear surface. Any p-dopant such as aluminum or boron may be used. Typically aluminum or aluminum alloy is deposited on the back surface and fired at high temperature in order to diffuse p-dopant into the silicon. The aluminum or aluminum alloy may be deposited by screen printing or vapor deposition. Typically, in a standard process for silicon solar cell fabrication, solderable busbars comprising silver or silver-aluminum paste are screen printed on the rear surface, then aluminum paste is screen printed over the entire back surface except areas covered by the busbars, then fired to remove solvent binders, harden the paste layers, and diffuse the aluminum p-dopant into the silicon.
The solar cell contacts must be formed, whereby a full area metal contact is made on the back surface comprising the p+-doped BSF and solderable busbars as described above, and a grid-like metal contact made up of fine “fingers” and larger “busbars” is formed on the front surface, typically formed by screen printing silver paste into a pattern of said fingers and busbars, then firing at high temperature to remove solvent binders, harden the pattern, and form ohmic contact to the cell. After the solar cell conductors are formed, multiple solar cells are then interconnected in series (and/or parallel) by flat wires or metal ribbons, and assembled into modules or “solar panels.” The finished solar panel product typically has a sheet of tempered glass on the front and a polymer encapsulation on the back to protect it from the environment.
Silicon is the most commonly used material for solar cell panel manufacturing. FIG. 1 shows the front side 10 having front side metal busbars 12 and metal lines 14 and the backside 20 having backside metal busbars 22 of a typical silicon solar cell. The metal busbars 12 and metal lines 14 on the front side of the solar cell preferably comprise a printed silver paste which is plated upon with the plating composition and method of this invention. The backside metal busbars 22 preferably comprise either silver paste or an aluminum-silver paste in contact with silicon. The remainder of the cell back side is preferably covered by a fired aluminum layer 38.
FIG. 2 shows a cross-sectional view of a typical silicon solar cell having an anti-reflective coating layer 32, an n-doped silicon layer 34 and a p-doped silicon layer 36. The silicon may be single crystalline or multicrystalline silicon, by way of example and not limitation. The metal lines 14 on the front side 10 collect the light-induced current. The front side busbars 12 collect current from the multiple metal lines 14 or “fingers.” The backside 20 of the cell typically has a set of busbars 22 similar to the front side; however, the backside 20 does not need to allow for transmission of light. The front side busbars 12 and backside busbars 22 allow for the connection of cells in series for modules. The backside busbars 22 contact the silicon substrate. The remainder of the back side is covered with a layer 38 of aluminum or aluminum alloy. The layer of aluminum or aluminum alloy 38 is generally applied by printing an aluminum or aluminum alloy paste on the backside of the solar cell and then baking the layer.
Competing factors must be considered in designing the front side metal pattern. The front side of the device must allow transmission of light so the metal traces should cover the smallest possible area in order to minimize shading losses. On the other hand, efficient current collection favors the coverage of the largest possible surface area since the sheet resistance of the front side may be relatively high (about 50 to 100Ω per square), leading to resistive losses if the coverage is too low.
A variety of methods may be used to form the front metal pattern, including screen printing of conductive paste, ink jetting, and electroplating onto a seed layer. One commonly used method is screen printing of a silver paste containing a glass frit, followed by a firing step at about 800° C. during which the paste burns through the anti-reflective coating, if present, forming a grid of metal paste lines and busbars. While this method provides conductive patterns with reasonably good electrical contact, conductivity and adhesion, performance could be further improved by the deposition of additional metal onto the grid, since the post-fired paste necessarily contains voids and non-metallic filler.
In another method used to form a front side conductive pattern, metal is deposited from a solution of soluble metal ions onto a pattern of lines and busbars formed in the anti-reflective coating. A variety of methods may be used to form the pattern, such as photolithography followed by etching, mechanical scribing, or laser imaging. Such a method is described in International Publication No. WO 2005/083799.
Deposition of a metal from a solution of soluble metal ions occurs by an electrochemical mechanism, where oxidation and reduction reactions take place. Defined broadly, there are three different mechanisms for depositing metal on a substrate from solution:    (1) Displacement, also known as galvanic, deposition is where deposition of a metal on a less noble metal substrate is accompanied by transfer of electrons from the less noble to the more noble metal, resulting in deposition of the more noble metal and dissolution of the less noble metal substrate. This method may be limited in that the deposit may be limited in thickness since deposition will normally stop when the less noble substrate is completely covered. Also a portion of the substrate will be consumed. Also, the deposited metal layer may be non-continuous since deposition of the more noble metal is accompanied by dissolution of the underlying less noble metal, leading to poor conductivity and adhesion.    (2) Electrolytic plating is where oxidation and reduction are induced, by means of an external source of electrical current. This method provides dense, high quality metal layers with fast deposition rates that are not limited in thickness. However, an electrical connection must be made to the substrate, with an anode present in the bath to complete the electrical circuit. Making good electrical contact with the cells may be problematic since it may result in breakage of the fragile silicon substrate.    (3) Autocatalytic, also known as electroless plating, deposition is where reduction of the metal ions is accomplished chemically by inclusion of a reducing agent in solution, where deposition only takes place on catalytically active surfaces. This method eliminates the need for electrical contact and an external power source. However, in practice this method suffers several drawbacks. Firstly, the process may be difficult to control since the solution is inherently thermodynamically unstable; spontaneous decomposition with precipitation of metal may occur unless great care is taken to optimize the system. This in turn limits deposition rates which may be very slow. In particular, autocatalytic silver plating solutions are well known in the art to be highly unstable.
To solve some of these problems, the prior art has suggested various methods of electroplating on photovoltaic devices utilizing, for example, light-induced voltage to effect metal deposition.
U.S. Pat. No. 4,144,139 to Durkee, the subject matter of which is herein incorporated in its entirety, describes a method for plating electrical contacts onto the surface of a solar cell by immersion of the cell in an electrolyte solution containing metal ions and exposing the surface of the solar cell to light such that plating of metal occurs on the anode surface of the device. The back (anodic) side is covered by a thick sacrificial layer of silver, such that silver dissolves from the anodic backside and deposits on the cathodic front side when the device is irradiated. A cyanide-containing silver plating solution is described. Although cyanide-containing silver electrolytes are well known to yield excellent plating results, the use of cyanide is not preferred due to safety as well as environmental considerations.
U.S. Pat. No. 4,251,327 to Grenon, the subject matter of which is herein incorporated by reference in its entirety, describes an electrolytic method for plating similar to that described in U.S. Pat. No. 4,144,139. In addition, this patent describes an arrangement where the anodic backside of the device is attached to the negative terminal of a DC power supply and the positive terminal of the power supply is attached to a silver electrode in the plating solution, such that when the cell is irradiated with light and the current is adjusted appropriately, neither deposition nor corrosion occurs on the backside. Again, a cyanide-containing silver plating solution is used. This arrangement is shown in FIG. 3. A negative aspect of this method is the requirement of attachment of an electrode to the cell, which may result in breakage of fragile silicon substrates.
U.S. Pat. No. 5,882,435 to Holdermann, the subject matter of which is herein incorporated by reference in its entirety, describes a process where a printed metallic front side pattern on a photovoltaic cell is reinforced by photo-induced deposition of a metal such as copper or silver. The back (anodic) side includes a printed sacrificial metal paste such that charge neutrality is maintained by dissolving of metal from the back side concurrent with deposition on the front side when the device is irradiated.
U.S. Patent Publication No. 2008/0035489 to Allardyce, the subject matter of which is herein incorporated by reference in its entirety, describes a method of plating electrical contacts on photovoltaic devices where the device is exposed to light while immersed in a silver plating solution comprising silver ions, at least one of a nitro-containing containing compound, a surfactant, an amino compound and at least one of either an amino acid or sulfonic acid. However, this electrolytic “light induced plating method” for metallizing photovoltaic devices is the same or similar to the method described in the previously described U.S. Pat. No. 4,251,327 patent and suffers from the same deficiency, namely the requirement of attachment to the cell of an electrode connected to a current source and anode.
U.S. Patent Publication No. 2007/0151863 to Morrissey, the subject matter of which is herein incorporated by reference in its entirety, describes a non-cyanide silver electroplating composition comprising silver in the form of a complex with hydantoin or substituted hydantoin, an electrolyte, and 2,2′-dipyridyl.
U.S. Pat. No. 5,601,696 to Asakawa, the subject matter of which is herein incorporated by reference in its entirety, describes a cyanide-free silver electroplating bath containing silver in the form of a complex with hydantoin or substituted hydantoin.
U.S. Pat. No. 4,126,524 to Hradil et al., the subject matter of which is herein incorporated by reference in its entirety, discloses cyanide-free silver electroplating solutions containing silver complexed with imides of organic dicarboxylic acids, such as succinimide.
U.S. Pat. No. 4,246,077 to Hradil et al., the subject matter of which is herein incorporated by reference in its entirety, describes cyanide-free silver electroplating solutions containing silver complexed with pyrrolidine-2,5-dione (succinimide) or 3-pyrroline-2,5-dione (maleimide).
U.S. Pat. No. 5,322,553 to Mandich et al. the subject matter of which is herein incorporated by reference in its entirety describes a cyanide-free electroless silver plating solution comprising a silver cation, thiosulfate, and sulfite. This patent claims a plating rate and solution stability superior to conventional silver plating solutions.
Electroless silver plating also suffers from several deficiencies. For example, the baths are well known to be highly unstable with decomposition occurring readily, causing loss of silver due to precipitation, limited bath life, and un-wanted deposition on surface regions that should remain free of metal. Also, plating rates are generally very slow under conditions necessary for suitable stability.
Faster plating rates can be obtained by electroplating, including light-induced plating as described in the prior art, in which an external power supply provides current to the devices. However, the attachment of an electrical connection can be problematic in that it can result in breakage of fragile silicon solar cells.
Therefore, it would be desirable to provide a plating method that is capable of the faster plating rates realized by electroplating without resulting in breakage of the silicon solar cells from the attachment of an electrical connection and that also minimizes the noted deficiencies of electroless silver plating.
The present invention addresses these deficiencies by using the specified plating solution and method. The improved plating solution of the invention is activated by light when used to plate metal on photovoltaic cells. The plating solution does not contain cyanide. No electrical contact with the device is necessary.